1. Field of the Invention
The present invention relates to a development device and an image forming apparatus such as a copier, printer, facsimile machine, plotter, or multi-function device.
2. Description of the Background Art
Electrophotographic image forming apparatuses such as copiers, printers, facsimile machines, plotters, multi-function devices, or the like typically include a development device and a transfer unit. The development device develops a latent image formed on a photoreceptor serving as a latent image carrying member into a visible toner image. The transfer unit transfers the toner image from the photoreceptor onto a recording medium (e.g., transfer sheet) to form an image on the recording medium.
Much-sought-after features of such apparatuses include compactness, high-quality imaging, and speed. In an image forming apparatus proposed in JP-2009-116198-A, by positioning a developer container separately from a developing section to visualize a latent image formed a surface of the image carrier and circulating the developer, the developing section can be made compact. In addition, by providing the developer container with an efficient agitator, the ability to mix and disperse the supplied toner into the developer can be improved. Thus, high-quality images can be attained even when the printing speed is increased. In this example, because the developing section is compact, this technique can be used for a development device including multiple stations (i.e., more colors) to increase the image quality.
A possible drawback of the more-compact development device described above is that, because the development device can be made more compact, the surface area of the actual developing section of the device shrinks, degrading the ability to disperse heat efficiently.
Heat generation is intrinsic to image formation. In electrophotographic image forming apparatuses, a toner image is formed on a recording medium through a charging process, an exposure process, a development process, a transfer process, and a fixing process. While these image forming processes are performed, for example, a motor, a lighting source, and a fixing device all produce heat. More specifically, in the developing section, heat is generated by a difference in linear velocity between a photoreceptor and a development roller, an eddy current generated by rotating the development sleeve around the magnet at high speed, and friction between the developer and a doctor blade while the accumulated developer is smoothed by the doctor blade. Thus, the development section itself generates heat.
Moreover, with this configuration, the temperature in the image forming apparatus is increased when printing is continuously performed, affecting the properties of the toner in the development device. As a result, operating problems, such as a decrease in the fluidity of the developer and toner coagulation, are apt to occur, which may cause defective image formation.
In the development device described above, in order to inhibit the temperature from increasing, external air is sucked into the device and circulated by a fan. However, in a configuration in which the developer container is provided separately from the developing section, because the developing section is compact, that is, the developing section has a small outer surface area, the cooling efficiency is limited. As a result, the temperature increase of the developer of the development device during driving may be greater than that of a known development device in which the developer container is not provided separately from the developing section. In addition, in order to circulate the external air in the development device with the fan, providing a circulation path is required, which hinders the ability to make the configuration compact.
An approach has been proposed in which, in order to cool the developer, the developer is conveyed by air, that is, external air whose temperature is lower than that of the image forming apparatus, so that the developer can be cooled. However, in this example in which the developer is cooled during transport, the cooling time is normally insufficient, and therefore the transport path is required to be lengthened. If the transport path is lengthened, then when the developer is conveyed by air, the decrease of the transport efficiency is caused, and thus the configuration is impractical. Therefore, this approach cannot solve the problem that the developer is not cooled sufficiently.
In view of the foregoing, there is market demand for a development device in which the developer container is provided separately from the developing section and which is capable of cooling the developer effectively and efficiently without lengthening the transport path.